Process of removal of sulphur compounds from hydrocarbon streams using adsorbents

ABSTRACT

The invention concerns a process for the removal of sulphur compounds from a hydrocarbon stream, especially a gaseous hydrocarbon gas stream, which process comprises contacting said gas stream with an adsorbent comprising at least a zeolite having a pore diameter of at least 5 Å to adsorb the sulphur compounds thereon, the adsorption procedure followed by a regeneration procedure of used, loaded adsorbent by contacting the said loaded adsorbent with a regeneration gas stream having a relative water humidity between 1-100% for certain steps of the regeneration and a water content of below 5 ppmV for other steps thus to replace the adsorbed sulphur compounds by water. Suitably the regeneration is followed by a dry regeneration treatment to finalize the regeneration. The process of the invention allows to send the sulphur compounds containing regeneration gas without further treatment of e.g. physical/chemical absorption for sulphur components concentration to an appropriate treatment for sulphur removal such as e.g. a Claus unit.

The present invention relates to a process for the removal of sulphur compounds from a hydrocarbon stream, especially a gaseous hydrocarbon stream, comprising said sulphur compounds, which process comprises contacting said gas stream with an adsorbent comprising a zeolitic adsorbent.

The invention further concerns a process for the regeneration of the said adsorbent loaded with sulphur compounds.

The removal of sulphur-containing compounds from hydrocarbon streams comprising such compounds has always been of considerable importance in the past and is even more so today in view of continuously tightening process requirements and environmental regulations. This holds not only for natural gas streams to be used for e.g. the preparation of synthesis gas or for residential use or to be transported as liquefied natural gas, but also for natural gas liquid streams, natural gas condensate streams as well as for crude oil derived refinery streams containing sulphur compounds.

Sulphur contaminants in hydrocarbon streams include hydrogen sulphide, carbonyl sulphide, mercaptans, sulphides, disulfides, thiophenes and aromatic mercaptans, which due to their odorous nature can be detected at parts per million concentration levels. Thus, it is desirable for users of such natural gas and refinery streams to have concentrations (typical specifications) of total sulphur compounds lowered to e.g. less than 20 or 30 ppmV or less than 50-75 mg S/Nm³, the amount of non-hydrogen sulphide compounds lowered to e. g. less than 5, or even less than 2 ppmV or less than 12 mg S/Nm³ or even less than 5 mg S/Nm³.

Numerous natural gas wells produce what is called “sour gas”, e.g. natural gas containing hydrogen sulphide, mercaptans, sulphides and disulphides in concentrations that makes the natural gas unsuitable for direct use. Considerable effort has been spent to find effective and cost-efficient means to remove these undesired compounds. In addition, the natural gas may also contain varying amounts of carbon dioxide, which depending on the use of the natural gas often has to be removed at least partly. Streams used and obtained in refineries, especially hydrogen containing streams obtained in hydrodesulphurisation processes and obtained in hydrocarbon reforming processes as well as obtained by partial oxidation of sulphur containing feed streams, often contain the sulphur compounds as described before.

A number of processes are known for the removal of sulphur compounds and optionally carbon dioxide from hydrocarbon streams. These processes are generally based on physical and/or chemical absorption, chemical reaction and/or solid bed adsorption. Physical and/or chemical absorption processes, often using aqueous alkaline solutions, usually are able to remove hydrogen sulphide and, when carbon dioxide is present, a large amount of the carbon dioxide, in some cases even complete removal of the carbon dioxide is obtained. However, the complete removal of sulphur compounds as mercaptans, sulphides and disulphides is much more difficult.

Chemically reacting processes in general are able to remove carbon dioxide and/or hydrogen sulphide without large difficulties; however, they suffer from the fact that they do not effectively remove mercaptans, sulphides and disulphides and often produce large amounts of useless waste (such as non-regenerable adsorbents). Regenerable solid bed adsorption processes are very suitable for the removal of the larger sulphur compounds such as methyl mercaptan, ethyl mercaptan, normal and isopropyl mercaptan and butyl mercaptan. For instance, WO 2004/039926 discloses such an adsorption process carrying out a regeneration gas stream with a water relative humidity less than 100% and in which the degradation/ageing of the zeolitic adsorbent is significantly reduced.

However, the regeneration of the adsorption beds is often a considerable problem. During the regeneration of the adsorbent, a considerable amount of regeneration gas is needed and has to be treated by further absorption processes generally using physical and/or chemical solvents such as in e.g. Selexol® and Purisol® processes representing important investment and being not commercially available everywhere in the world due to export restrictions in order to meet the specifications in terms of sulphur compounds for further use.

The present invention relates to a sulphur compounds removal process that does not show the drawbacks of the prior art processes. Furthermore, in the case of regeneration of adsorbent loaded with these lower alkyl mercaptans, hydrothermal ageing and the formation of carbon on the zeolite adsorbent(s) are particularly low.

The present invention relates to a process for the removal of sulphur compounds from a hydrocarbon stream, especially a gaseous hydrocarbon gas stream, comprising said sulphur compounds, which process comprises contacting said gas stream with at least an adsorbent comprising a zeolite having a pore diameter of at least 5 Å to adsorb the sulphur compounds thereon and to obtain a treated gas, the adsorption process is followed by a regeneration procedure of used, loaded adsorbent, characterized in that the regeneration procedure comprises the following steps:

a/ contacting the said loaded adsorbent with a first dry gas having a temperature within the range from 15-200° C. and a the pressure of 5-70 bara in a turn-around way by recycling the first dry gas at least partially and preferably completely through the adsorbent, i.e. with or without a purge of gas,

b/ then contacting the loaded adsorbent with a regeneration gas stream having a relative water humidity between 1 and 100% saturation, a temperature of 15-200° C. and a pressure of 5-70 bara being partially recycled through the adsorbent or not,

c/ and finally contacting the said loaded adsorbent with a second dry gas having a temperature within the range from 50-350° C. and a the pressure of 5-70 bara.

The gas streams in step a) and b) could be recycled with a compressor and could be temperature regulated by heat-exchange.

The contacting of gas in step a) and b) may be performed in the same flow direction as the contacting in the adsorption process.

The temperature of the zeolite adsorption procedure may vary between wide ranges, and is suitably between 0 and 80° C., preferably between 10 and 60° C., the pressure is suitably between 10 and 150 bara. The superficial gas velocity is suitably between 0.03 and 0.6 m/s, preferably between 0.05 and 0.4 m/s.

For the regeneration procedure, the regeneration gas stream to be used may be in principle each inert gas or inert gas mixture. Suitably nitrogen, hydrogen or a hydrocarbon gas stream, a mixture of saturated light hydrocarbons, preferably methane, possibly containing inert gases such as N₂, CO₂, Ar can be used, preferably a treated gas stream which is obtained by a sulphur removal process as described above.

In Natural Gas Processing units, the regeneration gas could be e.g. the Sales Gas (product gas) containing generally above 95% vol methane and below 5% ethane and heavier hydrocarbons, or the demethanizer overhead from the gas fractionation part of the Process unit or the residual gas (70% vol methane, 30% vol N2) from the Nitrogen rejection unit in case of LNG production or the boil off gas from LNG storages.

For the first and the third step of the regeneration procedure, the same dry gas can be used.

For the second step of the regeneration procedure, the gas which has a relative water humidity between 1 and 100% saturation, preferably between 1 and 50%, may be obtained by any suitable method. For instance, a dry gas may be mixed with a saturated gas, or a dry gas stream is saturated followed by an increase of the temperature. In some preferred embodiments, it can be the gas from the first step which contains some water in the specified range.

The third step of the regeneration procedure according to the present invention is finalized by regeneration with a dry gas stream. In this way the adsorption capacity is fully restored. One advantage of the invention is that the dry gas used for the final regeneration step can be recycled upstream the adsorption unit reducing thus the loss of valuable product as it contains only a very low quantity of sulphur compounds respecting the above mentioned specifications.

In those cases in which the temperature of the regeneration gas is above the condensation point of steam, the relative humidity is defined as the volume percentage of the water in the gas stream.

Very suitably the hydrocarbon stream to be treated is a gaseous hydrocarbon stream, especially a natural gas stream, an associated gas stream, or a refinery gas stream. Natural gas is a general term that is applied to mixtures of inert and light hydrocarbon components that are derived from natural gas wells. The main component of natural gas is methane. Further, often ethane, propane and butane are present. In some cases (small) amounts of higher hydrocarbons may be present, often indicated as natural gas liquids or condensates. Inert compounds may be present, especially nitrogen, carbon dioxide and, occasionally, helium. When produced together with oil, the natural gas is usually indicated as associated gas.

Sulphur compounds, e.g. hydrogen sulphide, mercaptans, sulphides, disulphides, thiophenes and aromatic mercaptans may be present in natural gas in varying amounts. Refinery streams concern crude oil derived gaseous hydrocarbon streams containing smaller or larger amounts of sulphur compounds. Also recycle streams and bleed streams of hydrotreatment processes, especially hydrodesulphurisation processes, may be treated by the process according to the present invention.

The process of the present invention may also be used for the removal of the sulphur compounds from liquid hydrocarbon streams as natural gas liquids streams, natural gas condensate streams and crude oil derived refinery streams, especially natural gas liquids streams and natural gas condensate streams. Natural gas liquids are well known in the art (see for instance The Petroleum Handbook, Elsevier, Amsterdam/London/New York, 1983, p 555) and contain hydrocarbons heavier than methane, usually contain C₃-C₁₂ compounds, often more than 50 weight % being C₄-C₁₀ compounds. Natural gas liquids (NGL) are suitably produced directly at the well head by separating the production stream from the subsurface formation at high pressure (usually between 40 and 90 bara) into a gaseous stream, an aqueous stream and a liquid hydrocarbon stream (the NGL stream). Cooling the gaseous stream usually results in a further amount of liquid products (condensates), mostly consisting of C₄-C₁₂ compounds, usually at least 50 wt % C₅₊ hydrocarbons. Suitable refinery streams are distillation fractions boiling in the naphtha, kerosene and diesel ranges (e.g. boiling ranges between 30 and 380° C.), as well as heavy gas oils and recycle oils (e.g. boiling between 250 and 450° C.).

The sulphur compounds which may be removed by the process of the present invention are in principle all compounds which are adsorbed by adsorbents comprising zeolites having a pore diameter of at least 5 Å. Usually the sulphur compounds are hydrogen sulphide, carbonyl sulphide, mercaptans, especially C₁-C₆ mercaptans, organic sulphides, especially di-C₁-C₄-alkyl sulphides, organic disulphides, especially di-C₁-C₄-alkyl disulphides, thiophene compounds, aromatic mercaptans, especially phenyl mercaptan, or mixtures thereof, preferably mercaptans, more especially C₁-C₄ mercaptans.

The invention especially relates to the removal of methyl mercaptan, ethyl mercaptan, normal- and iso-propyl mercaptan and the four butyl mercaptan isomers.

The starting hydrocarbon stream may contain any amount of sulphur compounds, but in general, the total amount of sulphur compounds will be up to 3 vol % based on total gas stream, is preferably up till 1.5 vol %, more preferably up till 0.1 vol %, still more preferably between 1 and 700 ppmV, most preferably between 2 and 500 ppmV. Higher amounts of sulphur, especially when it concerns mainly hydrogen sulphide, can be removed by the process of the present invention, but are more suitably removed by washing processes in which chemical and/or physical solvents are used.

The starting hydrocarbon stream can be a dry hydrocarbon stream (preferably having an amount of water≦5 ppmV, and more preferably≦1 ppmV) but, especially when it is a gaseous hydrocarbon stream, may contain a certain amount of water, preferably up to 1% mol and more preferably less or equal to 2,000 ppm mol. Especially in the case of natural or associated gas the stream will be saturated with water.

In the case that water is present in the hydrocarbon stream, a more efficient process is obtained when the water is removed before the removal of the sulphur compounds, preferably by adsorbing the water on a zeolite having a pore diameter of less or equal to 5 Å, preferably a pore diameter of 3 or 4 Å. In such preferred zeolites hardly any sulphur is adsorbed, only water is adsorbed. In general, the capacity of such zeolites is higher than larger pore zeolites. The amount of water to be removed may be small or large, but preferably at least 60 weight % of the water is removed, preferably 90 wt %, Very suitably water is removed to a level of less than 1% mol in the treated gas, preferably less than 100 ppmV, more preferably less than 5 ppmV.

The process according to the present invention preferably carries out an adsorbent comprising at least a zeolite dispersed in a binder, the zeolite(s) being preferably of a zeolite type A and/or a zeolite of type X. Such materials called also molecular sieves are commercially available.

A further improvement of the process according to the present invention is the use of adsorbent in the form of at least two beds, one bed comprising a zeolite having a pore diameter inferior or equal to 5 Å, preferably 3 or 4 Å, the second and, if present, the further beds comprising a zeolite having a pore diameter of at least 5 Å. It is also possible to have a “complex” bed, being an intimate mixture or a dry-blend of at least two different adsorbents.

In a preferred embodiment, the 1^(st) bed comprises at least a zeolite having the pore diameter of 5 Å that removes hydrogen sulphide, methyl- and ethylmercaptan while the 2^(nd) bed that comprises at least a 13 X zeolite removes all higher mercaptans and larger sulphur compounds. It will be appreciated that the above indicated beds can be applied in one single vessel, or may be spread over two (or even more) vessels.

In another preferred embodiment, there is another bed before the two beds mentioned above comprising at least a 3 Å and/or 4 Å and/or 5 Å zeolite to remove any water upstream of the sulphur compound removal; the said further bed may be incorporated into the above mentioned one or more vessels, or may be applied in an additional vessel. The advantage of using more than one vessel is that each vessel can be used at its most optimal conditions for adsorption as well as for regeneration.

The process according to the present invention may be carried out in a continuous mode, preferably using two or more adsorbers comprising zeolite, at least one adsorber in an adsorbing mode and at least one adsorber is a desorbing mode. Depending on the actual situation there may be combinations of two, three, four or even more adsorbers, one in absorbing mode, the others in different stages of desorbing mode.

Especially in the case of natural and associated gas, a considerable amount of the total amount of sulphur compounds is formed by hydrogen sulphide. Amounts of up to 10 or even 20 vol % or even more of hydrogen sulphide may be present. Further smaller or larger amounts of carbon dioxide may be present. Sometimes amounts of up to 10 or even 20 vol % or even more of carbon dioxide may be present. Suitably the gas stream comprises hydrogen sulphide and optionally carbon dioxide up till 2 vol % hydrogen sulphide, more preferably up till 0.5 vol % hydrogen sulphide.

In the case that larger amounts of hydrogen sulphide are present in the gas stream, it appears to be more efficient to remove the hydrogen sulphide (and at least part of the carbon dioxide) by means of a washing process, preferably prior to the adsorption process as defined above.

In a washing process, the gas stream is washed with a chemical and/or physical solvent, preferably an aqueous alkaline solution, more preferably an aqueous amine solution. The use of organic solvents or aqueous solutions of organic solvents for removing of so-called acid gases as hydrogen sulphide and optionally carbon dioxide and/or COS from a gas stream containing these compounds has been described long ago. See for instance A. L. Kohl and F. C. Riesenfeld, 1974, Gas Purification, 2^(nd) edition, Gulf Publishing Co. Houston and R. N. Maddox, 1974, Gas and Liquid Sweetening, Campbell Petroleum Series. Preferably a regenerable absorbent solvent is used in a continuous process.

On an industrial scale there are chiefly two categories of absorbent solvents, depending on the mechanism to absorb the acidic components: chemical solvents and physical solvents. Each solvent has its own advantages and disadvantages as to features as loading capacity, kinetics, regenerability, selectivity, stability, corrosivity, heat/cooling requirements etc.

Chemical solvents which have proved to be industrially useful are primary, secondary and/or tertiary amines derived alkanolamines. The most frequently used amines are derived from ethanolamine, especially monoethanol amine (MEA), diethanolamine (DEA), triethanolamine (TEA), diisopropanolamine (DIPA) and methyldiethanolamine (MDEA).

Physical solvents which have proved to be industrially suitable are cyclo-tetramethylenesulfone and its derivatives, aliphatic acid amides, N-methylpyrrolidone, N-alkylated pyrrolidones and the corresponding piperidones, methanol, ethanol and mixtures of dialkylethers of polyethylene glycols.

A well-known commercial process uses an aqueous mixture of a chemical solvent, especially DIPA and/or MDEA, and a physical solvent, preferably an alcohol, especially methanol or ethanol, cyclo-tetramethylene sulfone or its derivatives, or N-methyl pyrrolidone, preferably cyclo- tetramethylene sulfone. Such systems show good absorption capacity and good selectivity against moderate investment costs and operational costs.

Such washing processes according to absorption techniques perform very well at high pressures, especially between 20 and 90 bara. Preferably in the hydrogen sulphide removal step at least 90 wt % of the hydrogen sulphide based on total weight of hydrogen sulphide present in the gas stream is removed, preferably 95 wt %, especially hydrogen sulphide is removed till a level of less than 10 ppmV, more especially to a level of less than 5 ppmV.

The adsorption process according to the present invention allows to finish the sulphur compounds removal by concentrating the sulphur compounds in a small quantity of regeneration gas making it possible to mix it directly with the acid gas from the absorption unit for further treatment for sulphur recovery.

Other solutions could be drying and sulphur compounds condensation and then recovery of sulphur compounds in liquid phase. 

1. A process for the removal of sulphur compounds from a hydrocarbon stream, especially a gaseous hydrocarbon gas stream, comprising said sulphur compounds, which process comprises contacting said gas stream with at least an adsorbent comprising a zeolite having a pore diameter of at least 5 Å to adsorb the sulphur compounds thereon and to obtain a treated gas, the adsorption process is followed by a regeneration procedure of used, loaded adsorbent, characterized in that the regeneration procedure comprises the following steps: a/ contacting the said loaded adsorbent with a first dry gas having a temperature within the range from 15-200° C. and a pressure of 5-70 bara in a turn-around way by recycling the first dry gas at least partially and preferably completely through the adsorbent, b/ then contacting the loaded adsorbent with a regeneration gas stream having a relative water humidity between 1 and 100% saturation, a temperature of 15-200° C. and a pressure of 5-70 bara being partially recycled through the adsorbent or not, c/ and finally contacting the said loaded adsorbent with a second dry gas having a temperature within the range from 50-350° C. and a the pressure of 5-70 bara.
 2. A process according to claim 1, in which the contacting of gas in step a) and b) are performed in the same flow direction as the contacting in the adsorption process.
 3. A process according to claim 1, in which the hydrocarbon stream is natural gas, associated gas, a natural gas liquids stream, a natural gas condensate stream or a refinery gas stream.
 4. A process according to claim 1, in which the sulphur compounds are hydrogen sulphide, carbonyl sulphide, mercaptans, especially C₁-C₆ mercaptans, organic sulphides, especially di-C₁-C₄-alkyl sulphides, organic disulphides, especially di-C₁-C₄-alkyl disulphides, thiophene compounds, aromatic mercaptans, especially phenyl mercaptan, or mixtures thereof, preferably mercaptans, more especially C₁-C₄ mercaptans, the total amount of sulphur compounds preferably being up to 3 vol % based on total gas stream, more preferably up till 1.5% Vol, more preferably up till 0.1% Vol, still more preferably between 1 and 700 ppmV, most preferably between 2 and 500 ppmV.
 5. A process according to claim 1, in which the gas stream also comprises water.
 6. A process according to claim 1, in which the gas stream also comprises hydrogen sulphide and optionally carbon dioxide.
 7. A process according to claim 1, in which the temperature of the zeolite adsorption process is between 10 and 60° C., the pressure is between 10 and 150 bara, and the superficial gas velocity is between 0.03 and 0.6.
 8. A process according to claim 1, in which the temperature of the first and second steps of the regeneration process is between 100 and 350° C., and the pressure between 5 and 70 bara.
 9. A process according to claim 1, in which the adsorbent(s) comprises at least a zeolite dispersed in a binder.
 10. A process according to claim 1, in which the adsorbent is in the form of at least two beds, one bed comprising zeolite having a pore diameter inferior or equal to 5 Å, preferably 3 or 4 Å, such as a 3 Å or 4Å zeolite, the second and eventually further beds comprising at least a zeolite having a pore diameter of more or equal to 5 Å.
 11. A process according to claim 1, in which the regeneration steps are carried out at a superficial gas velocity of less than 0.20 m/s.
 12. A process according to claim 1, in which the regeneration gas stream for step b) of the regeneration is a gas stream obtained by saturating the stream with water at a temperature below the regeneration temperature.
 13. A process according to claim 1, in which the regeneration gas stream for the step b) of the regeneration procedure has a relative humidity between 1 and 50%.
 14. A process according to claim 5, in which the water is removed before the removal of the sulphur compounds.
 15. A process according to claim 14, in which the water is removed by adsorbing it on a zeolite having a pore diameter of inferior or equal to 5 Å.
 16. A process according to claim 15, in which the water is removed by adsorbing it on a zeolite having a pore diameter of 3 to 4 Å.
 17. A process according to claim 6, in which the carbon dioxide is contained in the gas stream in an amount up to 2 Vol % hydrogen sulphide.
 18. A process according to claim 6, in which the carbon dioxide is contained n the gas stream in an amount up to 0.5% Vol % hydrogen sulphide.
 19. A process according to claim 16, in which the hydrogen sulphide and part of the carbon dioxide is removed by means of washing the gas stream with a chemical and/or physical solvent.
 20. A process according to claim 19, in which the chemical and/or physical solvent is an aqueous alkaline solution.
 21. A process according to claim 19, in which the chemical and/or physical solvent is a aqueous amine solution.
 22. A process according to claim 7, in which the superficial gas velocity is between 0.05 and 0.40 m/s.
 23. A process according to claim 11, in which the regeneration steps are carried out at a superficial gas velocity between 0.02 and 0.15 m/s.
 24. A process according to claim 11, in which the regeneration gas stream(s) comprise nitrogen, hydrogen or a hydrocarbon gas stream.
 25. A process according to claim 24, in which the regeneration gas stream(s) comprises a treated gas stream treated according to the process. 